1. Field of the Invention
This invention relates to the field of trimming circuits, and particularly to poly fuse trim cells.
2. Description of the Related Art
Poly fuse trim cells are used in many applications in which it is necessary to permanently program the digital values of one or more bits. For example, an 8-bit digital-to-analog converter (DAC) might be employed to provide a correction voltage to a particular circuit. Eight poly fuse trim cells could be programmed to provide the eight digital input bits to the DAC which cause it to produce the desired correction voltage.
A typical poly fuse trim cell is shown in FIG. 1. A poly fuse F1 is connected in series with a transistor MN0 at a node 10, with F1 and MN0 connected between supply rails VDD and GND. In response to a TRIM control signal (preferably buffered with one or more inverters U1, U2), MN0 conducts a current sufficient to cause F1 to xe2x80x9cblowxe2x80x9d and become an open-circuit. A current source circuit 12 is connected to node 10; circuit 12 is made of a transistor MN1, which provides a small current I1 (such as 2-5 uA) in response of a bias voltage VB. An inverter U3 inverts the logic level at node 10 and presents it at its output, which serves as the output OUT of the trim cell.
In operation, when the fuse is intact, the current through F1 (I1) causes a very small voltage drop across F1, and makes node 10 about equal to VDD (xe2x80x9chighxe2x80x9d), with the output of inverter U3 producing a logic xe2x80x9clowxe2x80x9d at OUT. When the fuse has been blown open by asserting the TRIM signal, the current I1 pulls node 10 down to near GND (xe2x80x9clowxe2x80x9d), making OUT a logic xe2x80x9chighxe2x80x9d. In this way, the state of OUT is xe2x80x9cprogrammedxe2x80x9d by either blowing F1 open or leaving it intact.
One drawback of this approach is that, for cells having fuses that are to remain intact (i.e., cells programmed to permanently produce a xe2x80x9clowxe2x80x9d), the pull-down current I1xe2x80x94while insufficient to affect the logic level of node 10xe2x80x94still exists as a quiescent current IQ(=I1) that flows through F1 and MN1. This can pose a problem when a number of such trim cells are used in a low power application. For example, if 20 poly fuse trim cells are used on a particular integrated circuit, the current consumption just for these cells can be 40-100 xcexcA, which may be unacceptably high.
A poly fuse trim cell is presented which overcomes the problems noted above, reducing or eliminating the quiescent current of cells having intact poly fuses.
The present poly fuse trim cell includes a poly fuse and a transistor connected together at a first node; as before, the transistor conducts a current necessary to blow open the fuse in response to a control signal. The cell also includes a current source circuit, a switching transistor, and a two-input logic gate; the switching transistor""s current circuit is connected between the first node and the current source circuit. The logic gate is connected to the first node at one input, and to a reset signal at its second input. The output of the logic gate provides the output of the cell, and is also connected to control the switching transistor.
The current source circuit, logic gate and switching transistor form a latch. In operation, when the fuse is intact, the logic gate output is in a logic xe2x80x9clowxe2x80x9d state such that the switching transistor is off (assuming an N-type transistor). This prevents current flow through the current source circuit, thereby making the cell""s quiescent current zero when it""s fuse is intact. When the fuse is blown open, the logic gate output goes xe2x80x9chighxe2x80x9d upon the occurrence of a reset signal, which turns on the switching transistor and allows the current source circuit to pull down the first node. In this way, the current consumption of a cell or a plurality of cells is reduced when compared with the prior art cell, assuming that the poly fuses of at least some of the trim cells remain intact.